In the chemical industry, catalytic gas phase processes, such as oxidation, hydrogenation, dehydrogenation, nitration, alkylation are performed successfully in tube bundle reactors comprising packed beds. Tube bundle reactors of this kind are known, for example, from DE 2 207 166 B4.
The reaction, in principle, may be either endothermic or exothermic. The packed bed, essentially a granular type catalyst, is located in a vertically positioned, generally ring-like bundle of reactor tubes which are sealed at both ends and fixed in tubesheets. The reaction gas mixture (feed gas) is supplied to the reactor tubes through a reactor head which spans the respective tubesheet, and it is likewise discharged (as a product gas mixture) through a reactor head spanning the other tubesheet. The terms reactor and tube bundle reactor will be used synonymously in the description to follow.
Stable reaction conditions are established by circulating a heat transfer medium at constant temperature by means of a pump to cool and heat, respectively, the reactor tube, followed by cooling/heating in a heat exchanger which usually is put in shunt. Inside the reactor, the heat transfer medium is guided by horizontally extending suitable baffle plates so that it flows substantially in transverse direction against the reactor tubes in the respective sections between the baffle plates and is guided axially from section to section through the reactor. Taken as a whole, therefore, the heat transfer medium flows lengthwise through the reactor. Donut-type and disc-type baffle plates have proved to be especially suitable, in particular for great tube bundle reactors which nowadays often dispose of 30,000 and more reactor tubes. For uniform reaction conditions to be obtained in the best possible way in respect of all the reactor tubes along the flow path, the mast uniform distribution of the heat transfer medium should be aimed at within each plane perpendicular to the reactor axis. In the case of WO 2004/052524 A1 that is achieved by corresponding partial stream apertures in the baffle plates. This is enhanced by a variety of other means, such as mixers, turbulence generators, or additional flow guide means. Accessories required for operation of the tube bundle reactor, such as a pump, heat exchanger, and heat-up means normally are positioned outside of the reactor shell to which they are connected directly by the shortest possible connecting lines. Thus the heat transfer medium enters the reactor shell in the vicinity of one tubesheet and leaves it in the vicinity of the other tubesheet.
As regards guidance of the flow of the heat transfer medium in the reactor system it is known, e.g. from EP 1 080 781 B1, to divide a partial stream from the heat transfer medium stream withdrawn from the reactor. The partial stream subsequently is fed into a heat exchanger and returned to the reactor by a circulating means. With a variant, the partial stream coming from the heat exchanger is supplied to an annular conduit at the heat transfer medium exit end. Such a design is disadvantageous in that either an additional circulating means is required in the heat exchanger circuit or control means must be installed in the main conduit extending parallel to the heat exchanger so that the heat transfer medium proportion flowing through the heat exchanger circuit will be adjustable. The consequence of this choice would be a stronger circulating means and thus higher investment costs.
DE 2 207 166 B4 shows an embodiment which does not require additional means in the heat transfer medium circulating system. Here, a single circulating means transports a major portion of the heat transfer medium back into the reactor as the main stream, while a smaller portion is fed as a side stream into a heat exchanger. When cooled therein, the side stream is combined with the main stream coming from the reactor and directed into the circulating means where both streams are transported further and mixed. Additionally, mixers may be provided downstream of the circulating means.
For optimum operating results, each tube bundle reactor is adapted to the respective process. In this respect, controlling the temperature along the reactor tubes is of major interest. Often it is sufficient to carry out the reaction as an isothermal process. In that event a one-zone structure is all that is needed. However, if the reaction envisaged is more complex and if a plurality of different temperature regions are required for optimum reaction control along the reactor tube it is recommendable to provide additional annular conduits, bypasses, or even a multi-zone structure which would improve influencing the temperature profile along the reactor tube. Examples for this way of proceeding are to be found in DE 2 201 528 B4, DE 28 30 765 A1, DE 698 01 797 T2, WO 2004/052526 A1, or WO 2004/067165 A1.
The selection of the heat transfer medium to be used for influencing the temperature of such gasphase processes largely depends on the particular reaction temperature. If the agent used is not one that boils at the reaction temperature but instead one which maintains its liquid state, the substances chosen should have a vapor pressure as low as possible. Liquid molten salts are widely used for such applications; they usually consist of a mixture of alkali nitrates and nitrites. For the sake of simplicity, the terms salt and salt mixtures will be used synonymously below. A preferred salt mixture consists of 53% by weight of potassium nitrate, 40% by weight of sodium nitrite, and 7% by weight of sodium nitrate. Such a mixture forms a eutectic which melts at approximately 142° C. The working temperature in general lies between 200° C. and 550° C. Reaction with oxygen and thermal decomposition reduce the nitrite content, whereas the share of nitrate increases. These processes can be delayed by superpositioning nitrogen. And if the temperature is limited to approximately 450° C. the salt practically may be regarded as being thermally stable. The melting point of the salt mixture in the equipment rises as a consequence of the decomposition process. Variations occurring in the other characteristic physical values are not remarkable and hardly have any influence on the heat transmission properties of the salt. Application, generally, is limited to a maximum of 620° C. because from this temperature on the salt reacts more strongly with iron.
As already mentioned, most of the publications on the topic of how to control the heat transfer medium are concerned with the adjustment of the optimum temperature profile. Mostly, they relate only to the steady operating condition. Only a few publications deal also with non-steady operating conditions during start-up and shut-down of the equipment.
GB 310 157 presents a heat exchanger which disposes of a bypass including a control valve which opens automatically during the start-up procedure when the heat transfer medium still is relatively cold and highly viscous, whereby a great proportion of the heat transfer medium is guided past the heat exchanger.
In general, when a heat exchanger is to be operated with liquid salt, serving as the heat transfer medium, it is preferable to melt the salt which, at the start still is in solid state, in a recipient tank. At the same time, the heat exchanger should be warmed up to a temperature higher than the melting temperature of the salt so that, when being filled into the heat exchanger, the molten salt will not cool down and solidify at once. It is likewise possible to leave the salt in the reactor itself and let it set there when the reactor is placed out of operation. In this event, renewed melting must take place from top to bottom. Moreover, there must be enough space for expansion above the solid salt surface. When it undergoes phase change and as the temperature continues to rise, the molten salt occupies an ever greater volume. With the provision described, it can expand upwardly without restriction. If it were melted from the bottom it would find no room into which to expand. In that case the expanding salt could cause undue deformation or even cracking of the surrounding walls.
Thereafter, the salt must be caused to assume operating temperature. It must be kept in mind that, just having been molten, the salt still is rather highly viscous which means that it causes great flow resistance. The following statements refer to a tube bundle reactor whose temperature is controlled by liquid salt, as an example of the kind of heat exchanger mentioned.
Preheating a reactor by passing hot gas through the reactor tubes is known in principle, e.g. from DE 1 542 517 B4 which describes a reactor where the reaction takes place in reactor tubes filled with catalyst. The rector tubes are cooled by the reaction gas entering the space inside the shell of the reactor. This reactor is equipped with an additional duct leading into the gas entry region of the reactor tubes and serving, among others, for introducing unreacted, preheated gas when the reactor is put into operation.
U.S. Pat. No. 6,046,343 is directed to dust removal from catalyst particles in reactor tubes. Part of this process resides in preventing catalyst damage caused by moisture uptake during rinsing. That is to be achieved by passing hot air through the space inside the shell of the reactor to heat the same to at least 120° C., preferably, however, to a temperature between 140° C. and 200° C. It is described in the patent that a heating alternative resides in passing hot air through the reactor tubes. Once the catalyst inside the reactor tubes has been freed from dust, nitrogen is passed through the catalyst bed for rinsing, while it is heated to starting temperature at the outside of the shell first by heated air, then by high pressure vapor, and finally by a salt bath functioning as heat transfer medium for the reaction.
German patent DE 2 062 095 B4 describes heating of a tube bundle reactor for carrying out exothermic chemical reactions, the reactor being cooled by a heat transfer medium which is not specified in detail. As the operating temperature lies between 350° C. and 450° C., the preferred heat transfer medium which suggests itself to those skilled in the art is liquid salt. It is mentioned in the publication that the system in question sometimes must be turned off and then restarted, e.g. to replace the catalyst. Upon start-up, the reaction process does not set in until after a higher temperature has been reached. That requires corresponding preheating of the tube bundle reactor. It is possible, in principle, to use an additional heat transfer medium which is passed through the reaction chamber. It is said in DE 2 062 095 B4 that vapor of sufficiently high temperature was available only rarely, whereas hot air and flue gases required a lot of expenditure in terms of heating units and accessories or led to operational deficiencies. To overcome that problem, it is suggested that a heater means connected in parallel with the given circling system of the heat transfer medium should be arranged, in addition, outside of the reaction vessel and linked to the latter by means of a combined blocking and regulating member at two places where different pressures prevailed.
EP 1 166 865 A2 reverts to and characterizes this per se known concept of preheating the reactor by passing hot temperature gas through it and subsequently introducing liquid salt into the space inside the shell. More specifically, this patent relates to a method of starting-up a tube bundle reactor designed for circulating a heat transfer medium which has a melting point in the range between 50° C. and 250° C. towards the outer surfaces of the reactor tubes. For start-up, a temperature gas having a temperature in the range between 100° C. and 400° C. is introduced into the reactor tubes, thereby raising the temperature. Subsequently the heat transfer medium is circulated in heated condition around the outer surfaces of the reactor tubes. The heat transfer medium is a salt mixture of alkali nitrates and nitrites. Further explanations given relate to the temperature of the temperature gas at the outlet from the reactor; from which temperature onwards the heat transfer medium is allowed to be filled in; continued raising of the temperature; use of the method in a multiple zone reactor; and the preparation of (meth-)acrolein and/or (meth-)acrylic acid after the reactor has been started up according to the method described above.
In EP 1 166 865 A2 an embodiment is described of a relatively small dual-zone tube bundle reactor having an internal diameter of 4,000 mm, a reactor tube length in the space inside the shell of 6,500 mm, and a number of 9,300 reactor tubes. For that particular reactor, true, the method presented may not cause damages. For larger tube bundle reactors, however, including more than 30,000 reactor tubes, having a diameter of about 8,000 mm and a length in the same order of magnitude, further aspects must be considered.
In particular, it must be kept in mind that, with such huge tube bundle reactors, it will take some time before temperature equalization has taken place between those parts of the equipment in direct contact with the heat-up medium and those at a greater distance from the same. For instance, if a reactor is at ambient temperature at the beginning of the start-up phase and hot gas having a temperature of 400° C. is fed into its reactor tubes the reactor tubes experience more expansion than the reactor shell. That may cause inadmissible thermal stress, going so far as to damage or even destroy the reactor.
In this context the heat-up rate, too, is of interest, i.e. the temperature increase per unit time. A general introduction to this topic may be gathered from the book by Klaus H. Weber entitled “Inbetriebnahme verfahrenstechnischer Anlagen”, Springer-Verlag, 1997, 1st edition. The author describes the start-up of equipment as a small but important part of setting a plant into operation. The aim should be to heat all parts of the equipment as uniformly as possible so that they will not attain different temperature levels which would cause undue thermal stress. In the chapter on putting operating means systems, especially vapor and condensate systems, into operation it is stated that the heat-up rate of cold piping should be 5° C./minute at the most, which would correspond to 300° C./hour (° C./h). However, this is true only provided the pipeline is heated uniformly throughout its length. The aim is infinitely more difficult to achieve with a large tube bundle reactor where diameters may be from 6 to 8 meter and the length may be in the same order of magnitude. A start-up example is given of an overall plant where two reactors containing bulk catalyst and working in parallel are started-up at an admissible heat-up rate of 20° C./h (cf. bottom of page 309 of Inbetriebnahme verfahrenstechnischer Anlagen).
FIG. 3 of EP 1 166 865 A2 is a diagram in which the temperature curves for the two zones of a dual-zone reactor are plotted above time. From this diagram, one can gather that the temperature gas causes a heat-up rate of approximately 50° C./h.
Moreover, it is disadvantageous to continue to heat the reactor with gas when the melting temperature of the heat transfer medium has long been reached and the reactor already is filled with liquid heat transfer medium salt. Proceeding in this manner, first the gas must be heated up. In a second step, the heat must be transmitted to the inside wall of the reactor tube. Thus there are two steps of heat transmission. If, on the other hand, reactor heat-up is effected by electrical heat-up means which are directly connected to the reactor, as is the case with a reactor according to DE 2 207 166 B4, the electrical energy is transferred directly to the heat transfer medium salt, making the heat-up procedure more direct and faster. Furthermore, there are no thermal losses caused by ducts which extend between the intermediate tank, external heat-up means, and the reactor. Apart from that, EP 1 166 865 A2 does not mention any heat insulation at all. The externally located parts of the equipment, like the intermediate tank and heat-up means make it necessary to provide additional pumps. This is in contrast to DE 2 207 166 B4 according to which flow through the electrical heat-up means takes place due to its communication with locations of different pressures in the heat transfer medium circulating system, without need for an additional pump.
During normal operation between lower and upper operating points, temperature variations may occur in the heat transfer medium system and that may cause volume variations of the heat transfer medium. Such operational changes of the volume conveniently are accommodated by an expansion tank designed for volume variation. According to DE 2 207 166 B4 this problem is resolved in simple fashion in that the heat transfer medium may expand into an expansion tank connected directly to the circulating pump and cooler. The capacity of the expansion tank is exceeded when the temperature of the upper operating point is surpassed, excess volume will flow through an overflow pipe into a collecting tank or back into the recipient heat transfer medium tank. When the temperature drops below the lower operating point, the missing heat transfer medium volume is compensated by replenishing from the recipient heat transfer medium tank.
EP 1 166 865 A2, moreover, describes the heat-up process for a multiple zone reactor. Only one intermediate tank is provided for adjusting different temperatures in the various zones, and a partial amount out of that tank is fed to a separate heater. For its heat-up, this reactor system requires an expensive measuring, control, and regulating system for the volume flows and temperatures. Independent control of the volume flows and temperatures can be effected to a limited extent only. Furthermore, separate conveying means and many connecting lines are needed between the tube bundle reactor, the intermediate tank, and the heater. And they must be furnished with insulated heat tracing to limit heat losses. Nothing is mentioned about the start-up of other parts of the equipment, such as the cooler which does not begin to operate until after heat-up.
During shut-down of the reactor, in general, the problems regarding temperature differences and cooling rate are similar mutatis mutandis. But no relevant printed prior art is known.
What is known in the art, mainly are ways and means of temperature control of the heat transfer medium for steady operation. As regards known controls for non-steady temperature variations, only general ways of proceeding are mentioned.
Certain heat-up rates are given, e.g. the figure of 20° C./h quoted above from the cited book “Inbetriebnahme verfahrenstechnischer Anlagen”. With them, however, a person skilled in the art does not know at what points these must be observed and, therefore, fails to know when in fact they are surpassed. Heating-up or cooling of a reactor is not an isothermal event. Instead, it varies greatly both in time and place. Hot and cold rolls, respectively, during which heating and cooling occur rush very rapidly through the reactor. While practically no heat-up or cool-off takes place in front of and behind a roll.
The problem of parts of the reactor system possibly becoming damaged by thermal expansions which differ too much when temperature differences are too great, is hardly discussed at all. Indications of how to avoid such damages are missing entirely. Moreover, start-up and shut-down of accessory units directly coupled to the tube bundle reactor are not considered in the prior art.